1. Field of the Invention
The present invention relates to an improvement in a continuous shooting function in an imaging device such as a digital still camera. More particularly, the present invention relates to an imaging device, which performs continuous shooting, while moving a focused position, and also to an imaging method in such an imaging device.
2. Description of the Related Art
An electronic imaging device such as a digital still camera (hereinafter, refer to a digital camera) generally includes an autofocusing (AF) system, which focuses a lens, camera, etc., automatically. As an AF control method in such an autofocusing system, for example, a control method such as a contrast method (sometimes, referred to as a hill-climbing servo) is widely used. One example of the contrast method is disclosed in JP S39-5265B.
In this contrast method, an integration value of luminance differences of adjacent pixels or a high frequency component of picture signals from the picture signals obtained with respect to one filed or one frame is obtained, so as to use this value as an AF evaluation value showing a focused degree. In a focused state, the AF evaluation value is increased because the edge part of a subject is clear, whereas in a non-focused state, the AF evaluation value is decreased. In the AF operation, this AF evaluation value is sequentially obtained while moving a lens, and the lens is stopped at a position as a focused point in which the AF evaluation values is the maximum.
In general, accurate focusing is required for a device which images a still image such as a digital camera, compared with a device which photographs a motion picture such as a video camera. For this reason, in such a device, the AF operation is conducted in every photographing operation, or the focusing operation is constantly repeated in a recording mode.
However, when a plurality of subjects each located in a different position is displayed on a screen, or the contrast of a subject is low, an image focused on a position different from a position intended by a photographer may be photographed.
In order to solve such a problem, for example, JP H07-131698A discloses a technique which photographs the designated number of images in a focused position detected by an AF operation and positions in the anteroposterior thereof, so as to enable the photographing of a desired image. In this technique, when a plurality of subjects is located in different positions, a subject detected as the focused position in the AF operation is not always a desired subject of a photographer. More particularly, when a desired subject is located outside the photographed distance range, the photographed images do not include a desired image, as a result, the photographed images are all wasted.
Moreover, for example, JP2001-116979A discloses a technique which performs photographing a plural number of times with respect to the detected subjects located in different positions as the focused points. However, in his technique, since the photographing is conducted in the focused position selected by a camera, when a plurality of subjects each having a contrast higher than the contrast of a subject intended by a photographer is located, the subject intended by the photographer is not selected as a photographing target. For this reason, in this technique, it is necessary to confirm whether or not the photographing corresponding to a desired distance is performed after photographing.
On the other hand, for example, JP2005-277813A discloses a technique which selects an area on a screen desired for focusing, after photographing all images in a photographing distance range, so as to select an image focused on that area. In addition, for example, JP 2007-28486A discloses a technique which stores an image selected by a camera or a user in a memory card after photographing all images in a photographing range.
In this case, the shorter the subject distance or the larger the focal length, the narrower the range of the depth of field is obtained. Accordingly, in order to photograph by changing a focused position while maintaining a focused state, it is required that photographing is conducted a plural number of times.
The techniques disclosed in JP2005-277813A and JP2007-28486A can reduce errors when the number of pixels of a recording target is small, but the techniques require a significantly high-capacity buffer memory for storing an image when the number of pixels of an image sensor is large.
For example, it depends on the spec of an optical system, but a digital camera, which uses a 1/2.5 type image sensor having 8 million pixels to 10 million pixels, requires photographing of about 30 pictures from a close distance to infinity at the wide-angle side, and photographing of about 70 pictures from a close distance to infinity at the telephoto side. However, when using such an image sensor, even if a 1 G bit image buffer memory is used, the image sensor processes only about 10 images at one time.
Therefore, in order to obtain an image focused on a desired subject, it is ideal to photograph all images at focused positions from a close distance to infinity. However, considering the costs, the control of the photographing distance range is unavoidable in terms of limitation regarding the memory capacity and the processing speed.
In this case, if an image is stored with respect to each photographing while being compressed, it is not necessary to use a large-capacity buffer memory. However, a longer time for a process of each image is required, and the repeated photographing interval is increased. For this reason, it becomes difficult to sequentially photograph a moving subject. On the other hand, it is possible to solve a problem of time lag by using an ultra high-speed system, but it consumes a significant amount of power.
Moreover, JP2005-202064A discloses a technique which displays on a screen the position of the present focus lens and the focused range of that state before sequentially obtaining AF evaluation values. In this case, a photographer can modify the focused range according to the display of the position of the focus lens and the focused range of that state, and the blurring of the subject on the screen. After that, the AF operation is conducted in response to the half-pressing of the release button, and only one picture is taken in response to the full-pressing of the release button.
The above-described techniques are examples of focus bracketing photographing or a focus continuous shooting, and a technique which conducts photographing in the positions of a lens disposed anteroposterior of the focused point determined by a camera. More particularly, a technique, which photographs a plurality of images while changing a parameter originally used in exposure, is applied to focusing, this technique being know as focus bracketing photographing.
In addition to the above techniques, various techniques which photograph all images in a photographing distance area by changing a lens position at predetermined intervals, and store the images, so as to correspond photographing intent of a photographer after photographing. JP3934954B also discloses a technique which changes an operation such that a focusing operation is re-conducted when the focused state is maintained.